The present invention is directed generally to radiocommunication systems and, more particularly, to techniques for identifying and mapping areas of poor signal quality in a CDMA radiocommunication system.
A mobile phone network conventionally consists of a plurality of base stations arranged in a pattern so as to define a plurality of overlapping cells which provide radiocommunication support in a geographic area. Base stations in the network are located so as to provide optimal coverage of the mobile phone service area. The transmission pattern of a geographic arrangement of network base stations typically looks like a honeycomb of cells. Each base station with omnidirectional transmission in the network serves a roughly circular area with a diameter ranging from a few hundred meters to several kilometers depending on population density. Additionally, base stations may have adaptive antennas that cover only narrow sectors, thus producing xe2x80x9csectoredxe2x80x9d cells instead of circular cells. The mobile phone network typically only has a specified number of frequencies available for use by mobile subscribers. Therefore, to maximize use of the specified number of frequencies while preventing interference between adjacent base stations, each base station supports different frequencies than its corresponding adjacent base stations. When a mobile subscriber moves to the edge of a cell associated with a current servicing base station the mobile subscriber can be xe2x80x9chanded-offxe2x80x9d to an adjacent base station so as to enable call quality and signal strength to be maintained at a predetermined level.
Traditionally, radio communication systems have employed either Frequency Division Multiple Access (FDMA) or Time Division Multiple Access (TDMA) to allocate access to available radio spectrum. Both methods attempt to ensure that no two potentially interfering signals occupy the same frequency at the same time. For example, FDMA assigns different signals to different frequencies. TDMA assigns different signals to different timeslots on the same frequencies. TDMA methods reduce adjacent channel interference through the use of synchronization circuitry which gates the reception of information to prescribed time intervals.
In contrast, Code Division Multiple Access (CDMA) systems allow interfering signals to share the same frequency at the same time. More specifically, CDMA systems xe2x80x9cspreadxe2x80x9d signals across a common communication carrier by multiplying each signal with a unique spreading code sequence. The signals are then scrambled and transmitted on the common carrier in overlapping fashion as a composite signal. Each mobile receiver correlates the composite signal with a respective unique despreading code sequence, and thereby extracts the signal addressed to it.
The signals which are not addressed to a mobile receiver in CDMA assume the role of interference. To achieve reliable reception of a signal, the bit energy to interference ratio (Eb/Io) should be above a prescribed threshold for each mobile station. The bit energy of the signal is therefore adjusted to maintain the appropriate Eb/Io threshold level. However, increasing the energy associated with one mobile station increases the interference associated with other nearby mobile stations. As such, the radio communication system must strike a balance between the requirements of all mobile stations sharing the same common carrier. A steady state condition is reached when the Eb/Io requirements for all mobile stations within a given radio communication system are satisfied. Generally speaking, the balanced steady state may be achieved by transmitting to each mobile station using power levels which are neither too high nor too low. Transmitting messages at unnecessarily high levels raises interference experienced at each mobile receiver, and limits the number of signals which may be successfully communicated on the common radio frequency channel (e.g. reduces system capacity).
In a conventional CDMA system such as, for example, a CDMA system using the IS-95 standard, power control commands are transmitted from the base station to a mobile station so that a constant bit energy to interference ratio is maintained for each received signal at the base station. To accomplish this reverse link power control, the base station sends a power control bit 800 times a second over the forward fundamental channel to the mobile station. This power control bit informs the mobile station whether the mobile station should raise or lower its transmission power level so as to maintain a constant Eb/Io at the base station. A transmitted power control bit with a value of 0 indicates that the mobile station should raise power. A transmitted power control bit with a value of 1 indicates that the mobile station should lower power. In response to the transmitted power control bit, the mobile station adjusts the transmission power by 1 db increments on the reverse link. The base station then measures the Eb/Io ratio of the power adjusted reverse link signal and repeats the above process in an iterative fashion until the Eb/Io ratio reaches the specified level.
Measurement of the Eb/Io ratio provides an indication of either poor network coverage or high network interference conditions in a CDMA system. If poor network coverage exists at a given location of a mobile station then bit energy Eb will decrease (I will not change if the number of users remains the same) and thus the Eb/Io ratio will likely decrease. Furthermore, if high interference exists at a given location, the interference Io will increase and thus the Eb/Io ratio will likely decrease. An increase in the interference Io generally implies that the number of mobiles has increased, since Io is composed of the normalized interference from mobile stations in the same cell, the interference from mobile stations in adjacent cells, and the background noise. The component of the interference due to same cell mobile stations will generally predominate.
Poor network coverage can occur due to a number of conditions including poor network planning, localized terrain features, shadowing due to obstacles (e.g., buildings, trees) in the path of the mobile station-base station connection, and xe2x80x9cholesxe2x80x9d in network coverage due to the phenomenon of xe2x80x9ccell breathing.xe2x80x9d xe2x80x9cCell breathingxe2x80x9d occurs when a mobile on the edge of a cell transmits close to its maximum power to overcome interference from other mobiles in the cell and to communicate with the base station. When new mobiles enter the cell and are allocated a channel they will raise the overall interference level. Thus, the mobile station at the cell edge will have to raise its power further to maintain the required signal to interference ratio at the base station. However, due to maximum power limitations, the mobile station at the cell edge is unable to raise its power any further. Thus, mobiles in this situation are either handed off to another cell or another frequency or the call is dropped. The net effect of this process is that the cell border effectively shrinks. This cell shrinking due to high load can cause coverage holes between cells.
High interference conditions can occur when there are a large number of users in a cell in a CDMA network. These large number of users produce an unstable state where any single user must increase power to overcome interference from surrounding users. The increase in power of any single user causes an increase in the overall level of interference, which further causes other users to also raise their power. This process can result in a rapidly escalating state of congestion. High interference conditions can be managed by the network by balancing the requirements of all mobile stations sharing the same, common radio frequency channel, as already discussed above. However, high data rate services that require the transmission of bursty packets of data over the air can cause localized interference conditions that cannot be adequately managed by the network.
Often, as noted above, high interference or poor coverage conditions can persist in certain localities of a CDMA network in spite of the use of power control commands. These areas of persistent poor coverage or high interference can impair the quality of the mobile subscriber signal and also impair the ability of the network to avoid call dropping. In areas where poor coverage or high interference conditions exist, the quality of the phone signal will likely be degraded. Furthermore, in geographic areas of poor coverage or very high interference, the potential for call dropping exists.
Conventionally, network coverage and interference conditions are monitored through the performance of drive tests by network operator staff. To perform this monitoring, operator staff drive throughout the network and conduct and record call quality checks. This conventional monitoring technique, however, requires an inordinate amount of resources to survey the network. Such resources include extra monitoring equipment, extra staff to conduct the drive tests, and additional staff time to drive around and survey the network. Furthermore, the time delay between the actual time at which interference in a locality increases to a level that will have an adverse impact on call quality or system performance and the time taken to survey the network, tabulate the results, and implement changes in the network coverage, ensures a period of degraded performance to affected mobile subscribers.
Accordingly, it would be desirable to provide a technique for monitoring a cellular network that minimizes the time required to detect areas of poor network coverage or high interference and which further minimizes the necessity of operator intervention.
These desirable characteristics and others are provided by the following exemplary embodiments of the invention.
According to one exemplary embodiment of the invention a method of constructing a data representation indicating a signal quality associated with a location of a mobile station in a radiocommunications network is provided. The method of this exemplary embodiment comprises the steps of: selectively adjusting uplink or downlink transmission power between said mobile station and said network using power control parameters; providing a location of said mobile station based on at least said power control parameters; selectively constructing a data representation indicating signal quality using said provided location and at least said power control parameters.
According to a second exemplary embodiment of the present invention a method of constructing a map of signal qualities associated with locations of mobile stations in a radiocommunications network is provided. The method of this exemplary embodiment comprises the steps of: a) selectively adjusting uplink or downlink transmission power between said mobile stations and said network using power control parameters; b) providing locations of each of said mobile stations based on at least said power control parameters; c) constructing data representations indicating said signal qualities using said provided locations of each of said mobile stations and at least said power control parameters; and d) selectively repeating steps a) through c) to construct a map of signal qualities throughout at least a portion of said network.
According to a third exemplary embodiment of the present invention a method of constructing a map of signal qualities associated with locations of mobile stations in a radiocommunications network is provided. The method of this exemplary embodiment comprises the steps of: a) providing at least one parameter indicative of a signal quality associated with a location of a mobile station, wherein said at least one parameter includes transmission power control parameters; b) comparing said at least one parameter with at least one criteria to provide a comparison result; c) initiating a positioning request from said network based on said comparison result; d) providing a location of said mobile station based on said positioning request; e) constructing a data representation indicating said signal quality using said at least one criteria and said location; and f) selectively repeating steps a) through e) to construct a map of signal qualities throughout at least a portion of said network.